A known process for the production of bromostyrene comprises effecting dehydration of the respective bromophenyl methyl carbinol, or the respective bromophenyl ethyl alcohol. Another known process, exemplified in British Pat. No. 986,634, involves the dehydrobromination of either of the respective .alpha. or .beta. haloethyl mono-bromobenzene isomers.
The dehydrobromination of bromoethyl halobenzenes is effected by passing a mixture of the reactants together with an excess of steam over granular calcium sulfate catalyst. The use of calcium sulfate as a catalyst is also mentioned in U.S. Pat. No. 2,485,524 in vapor-phase dehydrohalogenations of substituted halo-benzenes. Other catalysts mentioned in the literature for dehydrohalogenations are calcium chloride, calcium oxide, calcium phosphate and various aluminas.
When applied to bromoethyl bromobenzene, all of these catalysts exhibit the serious disadvantage of causing the formation and deposition on the catalyst, of tarry substances, most probably comprising a polymer formed from the monomeric bromostyrene which is the primary product of the catalytic dehydrohalogenation. The formation of the polymer cannot be avoided, even when a large excess of an inert diluent. such as steam or nitrogen, is fed, together with the substrate into the reaction zone. The deposition of the polymer on the catalyst rapidly reduces its activity and thus frequent catalyst regenerations are required. If, to avoid this problem, the catalyst is discarded after relatively brief runs, the recharging with fresh catalyst makes the process comparatively expensive.
To eliminate the deposition of the tarry byproducts on the fixed bed of catalyst, a process is described in French Pat. No. 1,576,909, wherein the active catalyst as well as the reaction medium comprises certain mixtures of molten salts, and in particular mixtures containing bivalent metal chlorides, such as copper chloride, in combination with other salts which decrease the melting point of the salt mixture.
According to the above disclosure, it is possible to obtain, for example, high yields of vinyl chloride by the elimination of hydrogen chloride from ethylene dichloride; however, the said process produces only poor yields of monomers in the case of heavy, relatively non-volatile substrates. In particular, when the substrates according to the present invention are used, the conversion to the respective products is far from complete.
In U.S. Pat. No. 3,737,469 a process is described whereby molten salts are used very effectively as a reaction medium for the production of bromostyrene by passing through this medium the said substrate together with an aliphatic alcohol, and in particular methyl alcohol. The alcohol acts as an acceptor, or scavanger for the hydrogen halide which is eliminated from the substrate during the reaction.
It is believed that the fast reaction of the alcohol with the eliminated hydrogen halide has a pronounced beneficial effect on the yield of the desired product. The process can thus also be carried out at temperatures that are substantially lower than in the absence of the alcohol. This has the effect of increasing the selectivity of the reaction with respect to the desired product.
While results according to U.S. Pat. No. 3,737,469 are generally satisfactory, said process has certain insufficiencies, such as, for the need for special materials of construction to be used for the reactors containing highly corrosive molten salts. Another disadvantage of said process is in the high ratio of reactor-space to product produced per unit time (low space-time yields).
A further disadvantage is the formation of small quantities of saturated products such as ethyl bromobenzenes. These products have boiling points very close to that of bromostyrene, and it is therefore very difficult to separate them by fractional distillation. These saturated impurities are not polymerizable and act as chain transfer agents, thus interfering in the high molecular weight polymerization of the bromostyrenes.